Final Review:Contrast Flashcards
Difference between adjacent gray shades/image receptor exposures
Image Contrast
Two photographic factors allow detail to be seen:
-IR exposure
-Contrast
Dynamic Range
gray scale, Window Width
Range of brightness displayed on monitor
Receptor Exposure
How much is reaching the IR
Bit Depth
Shades of gray
Determines the dynamic range of image in digital systems (controls contrast)
Look up table
Short scale
-few shades of gray between lightest and darkest
-short/narrow dynamic range
High Contrast
Long scale
-more shades of gray between lightest and darkest
-large/ wide dynamic range
Low Contrast
Why is high contrast not necessarily a good thing in medial imaging
More shades of gray can reveal more information
Manipulating contrast
Compression or expansion of range of visible densities
-adjustment to kvp
-Window width left and right
Subject contrast is dependent on:
-kvp
-amount of irradiated material
-Type of irradiated material
What causes wider range of photon energy?
-greater penetrability
-greater range of exposures
-greater amount of scatter
-longer scale of contrast (film)
Higher kvp produces how much range of exposure
narrower range of exposures picked up by IR (yields a narrower histogram)
Excessive kvp’s reduce differential absorption and therefore limits the range of exposure values picked up by the IR. What can this lead to
Can lead to histogram errors
If the kvp does not match the subject contrast, what is being impacted?
Differential absorption
Increase in thickness and field size does what to the scatter and scale of contrast?
-increases scatter
-longer scale of contrast (film)
The amount of irradiated material is dependent on:
Thickness or part and field size
Type of irradiated material is influenced by:
Atomic number and tissue density
What does high atomic number and tissue density do to attenuation and photoelectric interactions
-greater attenuation
-more photoelectric interactions
Large differences in atomic number and tissue density of adjacent tissues
High Contrast
What is the primary controlling factor for the the patient
kvp
Why can’t the subject contrast be manipulated
Because it is the patient (tissue)
For a nongrid, what number should you not exceed?
80 kvp
Cannot be assessed by visualization
kvp
what scatter ration requires use of grids
greater than 50%
does focal spot effect contrast?
No affect of contrast
how does more matter affect contrast
decreases contrast
how does collimation affect contrast
Increased collimation increases contrast and decreases scatter
does the anode heel affect have any affect on contrast
no effect on contrast but effects receptor exposure
does SID affect contrast
SID only affects receptor exposure, not contrast
Air Gap Technique
6 to 8 inch gap to decrease scatter on IR. Increases contrast because scatter is not reaching IR.
Increased filtration decreases quantity but increases quality
Filtering out low energy photons
Increased filtration, decreased ESE
More to IR, less absorption. More transmission, filtering out low energy photons
Increased Filtration does what to beam energy and contrast?
Increased average beam energy and decreased contrast (film)
How is grid ratio and contrast related?
Directly. Increased grid ration increases contrast (more black and white because more clean up removing scatter)
Differences in density between two areas on an image
contrast
Long scale contrast
many shades of gray visible
short scale contrast
mostly blanks and whites present
Large differences in density
high contrast
small density differences
low contrast
Range of exposures that will produce radiograph in the diagnostic range of densities
Exposure Latitude
Latitude inverse relationship to contrast:
Wide latitude = low contrast
Narrow latitude = high contrast
Latitude relationships to gray scale:
Wide latitude= long gray scale contrast
Narrow latitude= short gray scale contrast
Example of long scale contrast:
Abdominal xrays or chest xrays
Example of short scale contrast:
Hand xrays (bony anatomy)
Look up table
Picture perfect image, manipulates values of gray levels, controlling contrast
is brightness in film or digital
digital (monitor)
Is density film or digital
Film
What are some of your subject contrast?
kvp
amount of irradiated material
type or irradiated materal
Differential absoprtion
Will it either transmit or absorb
Window width
Window leveling
Annotation
Any manipulation to an image
Post processing
Bit depth
Look up table
Histogram
Pre processing
